1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device and a method for fabricating the same to reduce a mask process and remove a photo active compound (PAC) and thereby improve productivity.
2. Discussion of the Related Art
With the progress of information-dependent society, the demand for various forms of display devices has increased. To meet such a demand, efforts have recently been made to research flat panel display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs), electro-luminescent displays (ELDs) and vacuum fluorescent displays (VFDs). Some types of such flat panel displays are being practically applied to various appliances for display purposes.
Of these, LCDs are currently the most widely used as substitutes for cathode ray tubes (CRTs) in association with mobile image display devices because LCDs have advantages of superior picture quality, lightness, slimness, and low power consumption. Various applications of LCDs are being developed as not only mobile image display devices such as monitors of notebook computers, but also monitors of TVs and laptop computers receiving broadcast signals and displaying images. Such a liquid crystal display device includes a first substrate provided with a thin film transistor array, a second substrate provided with a color filter array and a liquid crystal layer formed between the first and second substrates. The first substrate includes a plurality of pixel regions defined by gate lines and data lines that cross each other, a plurality of pixel electrodes formed in the respective pixel regions where data signals are supplied, respectively, and a plurality of thin film transistors to respectively drive the pixel electrodes. Also, the second substrate includes a color filter formed in each pixel region, a black matrix to prevent light leakage and a column spacer to maintain a gap between the first substrate and the second substrate.
The representative driving modes that are most commonly used for the liquid crystal display device include a twisted nematic (TN) mode in which liquid crystal directors are aligned to be twisted by 90° and are then controlled through application of a voltage thereto and an in-plane switching mode in which liquid crystals are driven by a horizontal electric field between a pixel electrode and a common electrode aligned in parallel on a substrate.
In particular, in the in-plane switching mode, pixel electrodes and common electrodes are alternately formed in an opening of the thin film transistor substrate and liquid crystal is aligned by a horizontal electric field generated between the pixel electrodes and common electrodes. An in-plane switching mode LCD device has a wide viewing angle, but drawbacks of low aperture ratio and low transmittance. In order to solve these drawbacks, a fringe field switching (FFS) mode LCD device is suggested.
The FFS mode liquid crystal display device includes a common electrode having a single electrode shape formed in a pixel region and a plurality of pixel electrodes having slit shapes formed on the common electrode, or includes a pixel electrode having a single electrode shape and a plurality of common electrodes having slit shapes, thus operating liquid crystal molecules by a fringe field formed between the pixel and common electrodes.
Hereinafter, a method for fabricating a general fringe field switching mode liquid crystal display device will be described with reference to the annexed drawings.
FIG. 1 is a sectional view illustrating a general fringe field switching mode liquid crystal display device. FIGS. 2A to 2E are sectional views illustrating a step of connecting a drain electrode to a pixel electrode in the liquid crystal display device of FIG. 1.
Referring to FIG. 1, the method for fabricating the general fringe field switching mode liquid crystal display device comprises forming gate lines (not shown), gate electrodes 10a, gate pad lower electrodes 10b and data pad lower electrodes 10c on a first substrate 10 using a first mask, and forming a semiconductor layer 13 including an active layer 13a and an ohmic contact layer 13b stacked in this order using a second mask. Also, the method comprises forming source and drain electrodes 14a and 14b, and data lines DL using a third mask, and forming first and second protective films 15a and 15b to cover the source and drain electrodes 14a and 14b. 
Also, the second protective film 15b is selectively removed using a fourth mask to expose the first protective film 15a corresponding to the drain electrode 14b, the gate pad lower electrode 10b and the data pad lower electrode 10c. Also, a common electrode 18 is formed on the second protective film 15b using a fifth mask. A third protective film 15c is formed to cover the common electrode 18 and the third protective film 15c is selectively removed using a sixth mask to expose the drain electrode 14b, the gate pad lower electrode 10b and the data pad lower electrode 10c. 
Also, a pixel electrode 16a connected to the drain electrode 14b, a gate pad upper electrode 16b connected to the gate pad lower electrode 10b and a data pad upper electrode 16c connected to the data pad lower electrode 10c are formed on the third protective film 15c using a seventh mask. Also, although not shown, a black matrix, R, G and B color filters and column spacers are formed on the second substrate. In order to perform these steps, the general liquid crystal display device is formed using twelve masks in total. Accordingly, the overall process is complicated and fabrication cost is increased.
Meanwhile, the general liquid crystal display device includes a second protective film 15b formed of a photo active compound (PAC) such that the second protective film 15b is interposed between the first and third protective films 15a and 15c, to reduce data load between data lines DL and pixel electrodes 16a and thereby decrease power consumption. In this regard, generally, the second protective film 15b formed of PAC, an organic insulating film, is thicker than the first and third protective films 15a and 15c formed of an inorganic insulating film. For this reason, time required for formation of the second protective film 15b is greater than time required for formation of the first and third protective films 15a and 15c, thus causing a deterioration in yield.
Furthermore, since the organic insulating film and the inorganic insulating film cannot be patterned through the same mask process, in the general liquid crystal display device, the first protective film 15a and the second protective film 15b are sequentially formed, as shown in FIG. 2A. The second protective film 15b is patterned through a fourth mask process to expose the first protective film 15a, as shown in FIG. 2B. Also, a common electrode 18 is formed on the second protective film 15b. 
Then, as shown in FIG. 2C, the third protective film 15c is formed such that the third protective film 15c covers the common electrode 18 and the exposed first protective film 15a. As shown in FIG. 2D, the first and third protective films 15a and 15c corresponding to a region where the second protective film 15b is removed, are patterned through a sixth mask process to expose the drain electrode 14b. Also, as shown in FIG. 2E, a pixel electrode 16a connected to the exposed drain electrode 14b is formed on the third protective film 15c. 
That is, as described above, the general liquid crystal display device entails a complicated manufacturing process and has a deteriorated yield, since the first and third protective films 15a and 15c, and the second protective film 15b are formed of different materials and thus cannot be simultaneously patterned.